The major general objective of these investigations is to study the physiologic maturation of the fetal lamb kidney and quantitate the contribution of the fetal kidney in compensating for various types of fetal distress. Specific investigations are designed to study: 1. The relative rates of maturation of glomerular filtration and renal tubular function. 2. The effects of induced metabolic acidosis on the fetus and the contribution of the fetal kidney in compensating for fetal metabolic acidosis. 3. The effects of intrauterine maturation on the responsiveness of the fetal kidney to antidiuretic hormone and fetal control of vasopressin secretion. 4. The role of the immature kidney in the etiology of hyperphosphatemia and tetany of the newborn infant and developing fetus. These studies will provide valuable information leading to better understanding of the response of the immature kidney and developing fetus to various physiologic and stressful situations. The long-term objective, therefore, is that an improved understanding of the potential adaptations and physiologic response of the animal fetus will enable a formulation of new or improved methods to increase survival in the human fetus and immature newborn. Further, these investigations will provide valuable physiologic data which may stimulate new concepts regarding intrauterine therapy and provide surgeons with valuable information as they develop techniques for fetal surgery.